MECHANICAL SEPARATION 303 The filling of filter presses and the maintenance of the proper pressure is done almost universally by pumps and montejus. Rotary pumps are most favored. Capacities for the square-leaved presses of one reputable maker are shown in the accompanying tables. The Sweetland filter is a frame press arranged so that the bottom can be lowered as a unit in order to remove the cake quickly by washing it out. The Vallez rotary filter consists of a number of circular leaves rotating about IK r.p.m. inside a tank which is kept under pressure. The effluent escapes through the shaft in which the leaves rotate. Washing, blowing and steaming are all possible with it. To dump the cakes the entire bottom of the body is unfastened. The filter appears to be popular in the beet-sugar industry. Brewery Presses.—In the filtration of beer and other malt beverages following defecation, and before bottling or barreling, plate presses are used, the plates being arranged vertically or horizontally. Filter mass coming in cakes or sheets and consisting of compressed cotton or cellulose is the filter medium. In using the filter mass it is broken into pieces and disintegrated in water. After this is done it is poured into the recesses of the plates, a screen in the latter holding the medium in place. Before filtering the filter mass is compressed with a powerful screw. In the vertical presses the screw • is a part of the press and the leaves are all compressed at the same time after the hinged cover of the press holding the screw is put on and bolted to place. (See Fig. 26.) Screws for compressing the medium separate from the press are also used and on the point of securing uniformity in the layers of medium they seem preferable as each leaf can be handled separately and watched to see that a layer of medium of uniform density is obtained. This requires some skill and attention, the danger being that the medium will not be sufficiently compacted in some places leaving channels through which the liquid will pass. At least two presses are required and the moment the filtrate shows any cloudiness the malt liquor is switched from the press in use and pumped through a fresh one. When this occurs the press is unbolted and the filter mass is removed to a mechanical washer where with the aid of hot water is freed of yeast and other substances. After the washing it can be used over again. Some brewers now incline toward the use of continuous rotary vacuum filters. Filter Aids.—Among these are calcium sulphate and carbonate, sawdust, flour, pulverized bone black, infusorial earth, etc. Where the amount of solids in the filtrate is small or where the solids are of such nature that they readily pass through ordinary filter media resort may be had to a filter aid to coat the medium and to prevent the filtrate from issuing from the press as a cloudy fluid. Filter aids are also used to increase the brilliancy of the filtrate and to assist in filtering solids which are adsorbed more or less readily by them. In such cases the aid after having adsorbed the solids or colloids is run with its liquor into the press with the same slight degree of trouble that is had with easily filterable materials. Filter aids may be employed in three ways. First a slurry may be made up with water and be passed through the press ahead of the material to be filtered. Second, particularly adaptable where a strong adsorptive effect is desired, the aid is mixed with the material to be filtered and both are passed through the press. In the final method enough of the aid is mixed with the material to be filtered to give a coat to the medium. Experience will determine the best way to use the aid and quite a field of research remains to be explored to determine the best aids for particular substances and the proper method of employing them.